SEXUAL REPRODUCTION. 269 



able it to swim rapidly, but apparently aimlessly, through the 

 water. In this form it is essentially a zoospore. (See ^ 306.) 

 After having moved about for a variable time and perhaps 

 increased its volume by growth, it loses its cilia, surrounds 

 itself again with a cell-wall, and resumes its ordinary mode of 

 life. In filaments of some multicellular algae a similar process 

 occurs. The contents of any cell may escape by the solution 

 of the cell-wall and become a zoospore. After swimming 

 about for a time the zoospore may come to rest, secrete a 

 cell-wall, and by repeated divisions in one plane produce an 

 individual similar to the parent. (See ^| 24.) It is evident 

 that such a method would give rise economically to a con- 

 siderable number of individuals. The process is essentially 

 the separation of the filament into pieces, each being the 

 contents of a single cell. 



372. Conjugation. — In other filamentous algae the cell- 

 contents, instead of escaping as a single zoospore, divide into 

 two or more zoospores. If, while these are still active, two 

 accidentally collide, the possibility of their adherence and 

 and the fusion of the two into one is conceivable. Such 

 fusion actually occurs among the zoospores of alga;, and is 

 (ailed conjugation, but in observed cases it follows a definite 

 method, and is not merely accidental. It is probable, how- 

 ever, that the first occurrence of conjugation was accidental, 

 and that it has become fixed and definite because those indi- 

 viduals in which it occurred with most certainty and regular- 

 ity thereby produced the most vigorous offspring. 



373. Imperfect sexuality. — In the alga Ulothrix, we have 

 a plant in which many of the processes just described still 

 occur. It produces zoospores of two kinds: (1) large ones, 

 with four cilia (C, fig. 301), formed in pairs in each cell (B) ; 

 (2) small ones, having two (rarely four) cilia, and arising 

 eight or sixteen from each mother cell (D). Both these sorts 

 of zoospores will grow, after a period of swimming, into new 



